This invention relates generally to fiber optic rotation sensors, and, more particularly to a fiber optic rotation sensor which is relatively unaffected by the environmental conditions surrounding it.
In the fields encompassing navigation, guidance and geophysical measurement, conventional gyroscopes leave much to be desired in their sensitivity in the measurement of rotation rates. In recent years, however, a dramatic development has taken place in optical technology with the invention of the laser gyroscope which is an example of a rotation sensor based on the Sagnac effect. The laser gyroscope combines the properties of the optical oscillator, the laser and general relativity to produce an integrating rate gyroscope. A good description of a laser gyroscope can be found in U.S. Pat. No. 4,013,365.
As pointed out in U.S. Pat. No. 4,013,365, the conventional laser gyroscope is a laser that has three or more reflectors arranged to enclose an area. The three reflectors, preferably mirrors, together with the light-amplifying material in the laser path, comprise an oscillator. In fact, there are two oscillators, one that has energy traveling clockwise and one that has energy traveling counterclockwise around the same physical cavity. The frequencies at which these oscillators operate are determined by the optical path length of the cavity they travel. In order to sustain oscillation, two conditions must be met: (1) the gain must equal to unity at some power level set by the amplifying medium, and (2) the number of wavelengths in the cavity must be an exact integer. If the first condition is to be achieved, the laser frequency must be such that the amplifying medium has sufficient gain to overcome the losses at the reflectors and the other elements in the laser path. In addition, the wavelength must be an exact integer for the path around the cavity. This last condition actually determines the oscillation frequency of the laser.
When the enclosed ring is rotated in inertial space the clockwise and counterclockwise paths have different lengths. The path difference in these two directions causes the two oscillators to operate at different frequencies. The difference is proportional to the rate at which the ring is rotating since path difference is proportional to inertial rotation rate. The readout of the gyroscope is accomplished by monitoring the frequency difference between the two oscillators.
Another rotation sensing device based on the Sagnac effect is the fiber ring interferometer of the type described in a publication by V. Vali and R. W. Shorthill in Applied Optics, Volume 15, No. 5, May 1976, pgs 1099 and 1100 in which a multi-turn fiber optic ring is used to increase the sensitivity of the device over earlier version of the Sagnac interferometer.
Even further substantially increasing the sensitivity of the fiber optic ring interferometer is the fiber optic rotation sensing interferometer of the type set forth in application Ser. No. 100,320, filed Dec. 5, 1979, now U.S. Pat. No. 4,323,310 issued Apr. 6, 1982, by Shaw et al, one of the inventors of this invention.
During utilization of the fiber optic rotation sensing interferometers of the past it was realized by the inventors of this invention that an important consideration in the practical operation of fiber optic interferometers for accurate rotation sensing at low rates is the fact that these interferometers or rotation sensors are environmentally sensitive. In other words, changes in temperature or strain in the fiber optic loop itself can result directly in errors in the measurement of rotation rates. Unless proper precautions are undertaken, optical non-reciprocity can occur in the optic fiber loop which renders the system environmentally sensitive. Consequently, it is essential in the utilization of fiber optic rotation sensors to eliminate this undesirable factor of environmental sensitivity.